Direct conversion in integrated circuit receiver architectures typically provides low power and low complexity compared to other architectures. This architecture converts the center of a received RF signal directly to DC in the first mixers. Salient blocks in a typical direct conversion receiver include a low noise amplifier (LNA), quadrature mixers that separate the received signal into in-phase (I) and quadrature-phase (Q) signal components, channel select filters, variable gain amplifiers and analog to digital converters (ADCs).
A major challenge in a direct conversion receiver, compared to a superheterodyne receiver, arises from the fact that the local oscillator (LO) signal, which drives the quadrature mixers, has to operate at the same frequency as the received signal in order to provide direct conversion. The LO signal can couple to different nodes in the receiver through parasitic capacitive and substrate coupling thereby providing an LO leakage signal. If the LO signal is generated remotely from the receiver, LO leakage can occur through bond wire coupling, as well.
LO leakage manifests itself in a variety of problems. If there is LO leakage to the receiver input, this signal is processed through the LNA and quadrature mixers to provide a spurious DC offset voltage at the mixer output. If this DC offset voltage is time invariant, it may filtered. However, since the amount of LO leakage is typically not constant (i.e., time variant) and gain through the LNA may vary over time, schemes to filter this DC offset voltage often prove ineffectual.
Additionally, if the LO leakage is radiated from the receive antenna and then reflected back from moving objects, it also causes time variant DC offsets, which are difficult to filter. A DC offset voltage of only a few millivolts at the mixer output may cause the receiver to saturate when a typical voltage gain of 40 dB to 70 dB is implemented at baseband frequencies. Even if the receiver is not saturated, the offset causes a reduced available dynamic range of the ADC.
Intermodulation distortion of the receiver describes its susceptibility to interfering signals. Both the LNA and the quadrature mixer produce second and third order intermodulation distortion. The second and third order intercept points (IP2, IP3) quantify a receiver's ability to maintain its fidelity in an environment of interfering signals. These intercept points are often referred to the receiver's input wherein the terms IIP2 and IIP3 are used. Increasing LO leakage causes IIP2 of the receiver to degrade. Therefore, LO leakage causes receiver degradation in the form of DC offset and IIP2. See, for example, “IIP2 and DC Offsets in the Presence of Leakage at LO Frequency,” by I. Elahi, K. Muhammad and P. T. Balsara, Express Briefs, IEEE Transactions on Circuits and Systems II: Analog and Digital Signal Processing, Volume 53, Issue 8, August, 2006, Pages: 647-651.
Accordingly, what is needed in the art is an effective way to suppress the effects of LO leakage.